Provided is an anticancer agent sensitivity-determining marker whereby the treatment responsiveness of individual patients can be discerned. Also provided is a new cancer-treatment means using said marker. The provided anticancer agent sensitivity-determining marker comprises at least one of the following molecules and/or metabolic substances that said molecules are involved in: a molecule, the main body or a fragment of which is detected as an anion at m/z between 149.05 and 149.06; a molecule detected as an anion at m/z between 152.99 and 153.00; a molecule detected as an anion at m/z between 724.34 and 724.35 (using a mass spectrometer); glycerol 3-phosphate; dihydrobiopterin; GABA; lactic acid; asparagine; aspartic acid; 2-methylbutyroylcarnitine and 1-methyladenosine; and glutathione.
